1. Field of the Invention
The invention relates to a process for preparing a silylated silica, and to the silica prepared thereby.
2. Background Art
It is known from EP 0 686 676 to Wacker Chemie to hydrophobicize silica using (1) a volatile silylating agent and (2) a liquid, relatively nonvolatile silylating agent, by adding at least one of the silylating agents in the form of a very finely divided aerosol. The advantage of this procedure is the high thickening effect of the silica produced by this process.
One disadvantage of this procedure is the complex reaction regime: it is necessary to operate with at least two different silylating agents, of which one is to be applied as a liquid and the other as vapor. This necessitates high process-related expenditures, for example requiring cost-intensive apparatus and process control. The process described in the patent achieves yields which are well below 100%.
Existing processes for coating silica with organosilicon compounds are based, according to the state of the art, on the controlled addition of silylating agent. A precondition for controlled addition is sufficient reactivity of the silylating agents. This is generally the case when the silylating agents have reactive groups on the silicon atom, such as halide radicals (Cl, Br), alkoxy radicals (OR) amino radicals (—N—R), or silazane radicals (—N—Si—). Hydrophobicizing with reactive silylating agents is a costly approach, since the abovementioned silylating agents are expensive to prepare. Secondly, as a consequence of the high reactivity of the silylating agents, they may interreact in an uncontrolled fashion during the course of the main reaction, leading to unwanted side reactions and side-reaction products. A further serious disadvantage of the above silylating agents is the emission of elimination products from the functional group, such as hydrogen halides, alcohols, amines or ammonia. This emission fosters corrosion problems and hence requires additional costs for special materials and increased cleaning operations. The emissions also create environmental pollution and/or complex waste-gas and waste-water purification operations, which are again costly.
It would be desirable to overcome the disadvantages of prior methods of treating silica, in particular, to avoid the use of highly reactive silylating agents with their associated problems and costs. It would be further desirable to provide a uniform silica product having a homogeneous layer of silylating agent.